Muay Thai question

This is an esoteric question, but one thing I've noticed about Muay Thai fighters, both from Thailand and overseas, amatuer and professional is how surprisingly skinny their legs and calves are and how undeveloped their lower body seems proportionally to their upper body.

I know that muscle size and tone definitely does not automatically guarantee adequate muscle strength, but if you look at boxers, MMA guys, rugby/soccer players and so on who all need strength, endurance and a fair bit of agility in their lower body, their legs look proportional and "normal" in comparison to their upper body; even the small, lean people in those disciplines who aren't very bulky upstairs don't have legs that seemingly resmble match-sticks, whereas in Muay Thai, it looks like the norm in all weight-categories.